A combustion chamber 1' of the prior-art pulse combustion apparatus is illustrated in FIG. 4. The combustion chamber 1' has a generally-oval vertical cross section as viewed from either of opposed sides thereof, as shown in FIG. 4. The combustion chamber has an inlet 3' and an outlet 4'. The inlet and the outlet are aligned with each other, and have a central space of the chamber 1' between them. A mixing chamber 10' is located on the upstream side of the combustion chamber 1'. The mixing chamber 10' has check valves V.sub.2. A flame trap 9 is located between the mixing chamber and the combustion chamber. A gas distributor 15' connects with the mixing chamber. Fuel gas is supplied through the gas distributor 15' into the mixing chamber 10'. Air supplied opens the valves V.sub.2 and enters the mixing chamber. The fuel gas and the air are mixed together in the mixing chamber. The fuel/air mixture flows through the flame trap 9, and enters the combustion chamber 10' from its inlet 3'. The mixture is ignited therein. The burned gas comes from the chamber 1' through its outlet 4', and flows through a tail pipe 17'.
Initially the air/fuel mixture supplied into the combustion chamber 1' is ignited in a forced manner, or by a spark plug (not shown in FIG. 4). And, once combustion starts normally in the chamber 1', subsequent air/fuel mixtures supplied into the chamber 1' need not be ignited in a forced manner, but ignite themselves. This "self-ignition" is caused by the possibility that a portion of the burned gas which has flowed into the tail pipe 17' may return into the combustion chamber 1' or by the possibility that a portion of the burned gas may not come from the chamber 1', but may remain therein. Such a portion of the burned gas ignites the subsequent air/fuel mixture.
It is to be noted that the air/fuel mixture prepared in the mixing chamber 10' needs further mixing so as to have a sufficient degree of uniformity of mixture to be normally ignited by the spark plug or to normally ignite itself, in the combustion chamber 1'. However, since, as mentioned above, the inlet 3' and the outlet 4' of the combustion chamber 1' are aligned with each other, it is natural that the mixture supplied into the chamber 1' is disposed to pass straight through the chamber 1' into the tail pipe 17'. Thus, it is possible that the mixture may be hardly mixed in the combustion chamber. Therefore, it is possible that the mixture may not be ignited normally or may not ignite itself normally.
Also, in the prior-art construction of FIG. 4, it is possible that a portion of the burned gas may flow upstream (i.e., in the left-hand direction in FIG. 4) and enter the flame trap 9'. In particular, such a possibility is great when highly ignitable gas, such as city gas, is used. In such a case, that is, if a portion of the burned gas flows upstream into the flame trap 9', it is natural that a subsequent air/fuel mixture may not flow smoothly through the trap 9' into the combustion chamber. It may result in an insufficient amount of air/fuel mixture being supplied into the combustion chamber.